Gas tube overvoltage protectors are widely used for the protection of electrical equipment from overvoltage conditions which may be caused by electrical surges, lightning, high voltage line contact, and the like. The gas tube is hermetically sealed and usually contains ionizable gas at a pressure lower than atmospheric pressure. At least two electrodes are provided, suitably spaced apart at such a distance that when a voltage connected to the two electrodes reaches a predetermined value, spark-over occurs, the gas ionizes and current flows through the tube.
The gas tube typically is connected to other devices in a fail-safe arrangement to meet various contingencies that may be imposed by a plurality of foreseeable problems. Illustrative of the types of fail-safe protection designs are U.S. Pat. Nos. 3,254,179; 3,281,625; 3,340,431; 3,396,343; 4,150,414; 4,320,435; 4,303,959 and 4,394,704.
For example, in case of a sustained power overload, as where a power line has come in continued contact with the protected line, a concomitant sustained ionization of the gas within the tube is produced. The resultant heavy currents through the tube will cause overheating which, in some cases, could destroy the overvoltage protector and constitute a fire hazard. A common approach to this problem is to employ elements which fuse in the presence of such overloads such as metallic or non-metallic fusible material and provide either a permanent short circuiting of the arrester directly, or function to release another mechanism, e.g., a spring loaded shorting member, which provides the short circuit connection (commonly, the arrester electrodes are both shorted and grounded). The presence of the permanent short across the tube to ground provides a low resistance path to ground to prevent further heating and to limit the voltage at the protected equipment terminals for safety requirements. This ground condition serves also to flag attention to that condition thus signalling the need for its inspection or replacement.
Should a gas tube develop a leak, the ingress of air may cause the spark-over voltage of a tube to increase to an unacceptable level. One method to protect against transient over-voltages, thereby avoiding any damage to the protected lines or equipment, is to electrically connect in parallel with the gas tube discharge path a back-up air gap. The air gap is designed to spark-over at a voltage ("the spark-over voltage") above that of the gas tube across which it is connected and below some critical maximum voltage. For this reason the spacing between electrodes of the air gap must be closely controlled and tested so that spark-over does not occur at a voltage value close to that of the gas tube across which it is fitted. Also an air gap spark arrester assembly is subject to particulate contamination which can affect the value of the spark-over voltage. In addition, since the air gap often is constructed as an integral part of the gas tube assembly, the air gap cannot be tested until the assembly is built. Thus, if the air gap is found to be defective, the gas tube is disassembled and reassembled with a new air gap. This can be a slow and costly process, particularly in large scale manufacturing.